The development of renewable energy sources has revitalized the need for large-scale batteries for off-peak energy storage. The requirements for such an application differ from those of other types of rechargeable batteries such as lead-acid batteries. Batteries for off-peak energy storage in the power grid generally are required to be of low capital cost, long cycle life, high efficiency, and low maintenance.
One type of electrochemical energy system suitable for such an energy storage is a so-called “flow battery” which uses a halogen component for reduction at a normally positive electrode in discharge mode, and an oxidizable metal adapted to become oxidized at a normally negative electrode during the normal operation of the electrochemical system. An aqueous metal halide electrolyte is used to replenish the supply of halogen component as it becomes reduced at the positive electrode. The electrolyte is circulated between the electrode area and a reservoir area. One example of such a system uses zinc as the metal and chlorine as the halogen.
Such electrochemical energy systems are described in, for example, U.S. Pat. Nos. 3,713,888, 3,993,502, 4,001,036, 4,072,540, 4,146,680, and 4,414,292, and in EPRI Report EM-I051 (Parts 1-3) dated April 1979, published by the Electric Power Research Institute, the disclosures of which are hereby incorporated by reference in their entirety.
Zinc-halide flow batteries have the possibility to become a cheap source of energy storage. Restrictions holding zinc flow batteries from becoming more widely used are energy density and lifetime. A general zinc-halide flow battery includes two electrolytes separated by a membrane. This is partially to increase efficiency, but it also is intended to increase stability of the electrolyte components. Unfortunately, the membrane is generally the first component of the system to degrade, and thus, is a limiting factor deciding the lifetime of the flow battery. The removal of this membrane has the potential to increase energy efficiency and lifetime of the system dramatically. However, there are many factors to consider in the removal. Electrochemical deposition of zinc, without any plating enhancers, may occur in a highly non-uniform manner. Zinc has a propensity to plate in dendrites. This can be detrimental to a battery system for many reasons, since increased deposition correlates to increased electrical storage. As such, there is a need for improved deposition uniformity.